The present invention relates in general to automotive sound systems and, more specifically, to a microprocessor-controlled system having access to system parameters while the microprocessor is primarily in a powered down state.
Today's advanced audio systems typically include a microcontroller which supervises the operation of the system. The use of a microcontroller gives the system designer a greater amount of flexibility in implementing new system functions. For example, microcontrollers facilitate various features such as electronic tuning, digital displays, search functions, and digital inputs. It is common practice to store the system operating parameters used by the microcontroller in a keep-alive memory when the car ignition switch is powered off and to restore these parameters when the ignition is again powered on.
Popular input devices for microprocessor controlled audio systems include a keypad matrix and a continuously rotatable electronic control knob. With these input controls, there is no longer a one-to-one correspondence between the setting of an audio parameter, such as volume, and the location of an input device as is the case with analog devices (e.g., potentiometers). Thus, an operator or driver is typically unable to examine a control setting on the audio system to determine a level prior to turning on the car ignition switch.
While it is desirable to retain operating parameters such as tuned station and volume while the ignition is off, problems can arise when the last settings which are used are disagreeable to a new operator such as where a different driver enters the automobile. The radio may have been turned off while at a high volume which might startle a user when the audio system is again turned on.
Microprocessor-controlled audio systems also feature the use of the microprocessor (or an associated device) to keep track of and display time-of-day on the system display panel. Typically, audio systems equipped with a time-of-day clock display the time even when the audio system power switch is turned off. However, such systems require that the car ignition switch be turned on (or to its accessory position) in order to provide a time-of-day display, which can be inconvenient.
Prior art microprocessor-controlled audio systems have failed to solve the foregoing problems due to the fact that audio system functions, such as control input and the display, are inactive while the car ignition is off. In particular, the microcontroller typically enters a halt state when the ignition switch is off in order to conserve battery power. A typical microcontroller consumes 15 milliamps or more of current during operation. This is too high a current to allow audio system normal operation when the battery is not being recharged by engine operation. By entering a halt state in which no further instructions are executed and in which the microcontroller clock oscillator is halted, current consumption by the microcontroller is typically reduced to less than one microamp. In prior art systems, the microcontroller is reset and enters its operating state when the ignition is again turned on. Although battery drain is minimized according to the prior art system, there is no operator access to system functions when the system is off and the foregoing problems are experienced.